1. Field of the Invention
The present invention relates to an arrangement and a method for acoustically determining the temperature of a fluid, and in particular for acoustically determining the temperature of a gas of unknown composition.
2. Description of the Prior Art
Conventional, non-acoustic, temperature sensors, such as thermocouples, thermistors and resistance thermometers, operate by monitoring their own temperatures, for example by monitoring temperature induced changes in electrical properties of the material of which they are constructed. Therefore, such sensors generally operate independently of the composition of the gas whose temperature is being monitored. Moreover, conventional temperature sensors generally have a relatively long time constant (of the order of a few seconds) and therefore are generally insensitive to rapid temperature variations.
Acoustic temperature sensors are known which operate by measuring the propagation time of an acoustic wave, typically ultrasound, through a fluid of known composition, from which the acoustic velocity V in the fluid is obtained according to the known relationship between propagation time t and propagation length L, which is expressed by the equation:
xe2x80x83V=L/txe2x80x83xe2x80x83(1)
The temperature of the fluid can then be determined, for example utilizing the known relationship as expressed by the equation (2) below. Acoustic temperature sensors have a relatively short time constant (on the order of a few milliseconds) and therefore effectively measure an instantaneous temperature of the fluid. This makes them well suited for measuring rapid and transient temperature changes in a fluid system, for example induced by corresponding rapid and transient pressure fluctuations within the fluid. However, since the acoustic velocity also depends on the composition of the fluid through which it propagates, acoustic temperature sensors are sensitive to the composition of the fluid, the temperature of which they are being employed to determine.
Known acoustic temperature sensors include means for monitoring the acoustic velocity in the fluid, the temperature of which is to be determined, and for producing an output signal related to the monitored velocity. A processor is coupled to this means to receive the output signal and to calculate the temperature of the fluid based on the known relationship between the fluid temperature T and the acoustic velocity (for example obtained utilizing the equation (1) above), which is expressed by the equation:
T=kV2xe2x80x83xe2x80x83(2)
where k is a constant related to the composition of the fluid.
Typically, the means producing the output signal is formed by an acoustic signal transmitter, a complementary acoustic signal receiver located relative to the transmitter to receive an acoustic signal from the transmitter after propagating a predetermined distance through the fluid and a timing device coupled to the receiver which determines the propagation time of an acoustic signal between the transmitter and the receiver in response to the receipt of the acoustic signal by the receiver, and which supplies a signal indicative of this propagation time to the processor, where it is employed in the calculation of the fluid temperature.
An object of the present invention is to provide a method and an arrangement for acoustically determining an instantaneous fluid temperature, which are insensitive to rapid temperature fluctuations.
The above object is achieved in accordance with the principles of the present invention in a method and an arrangement for acoustically determining an instantaneous fluid temperature wherein an acoustic velocity of a fluid is monitored and an output signal related to the acoustic velocity is generated, and wherein a conventional temperature sensor is employed for sensing a fluid temperature, which generates an output signal related to the fluid temperature, and wherein these output signals are processed to determine the instantaneous fluid temperature therefrom.
By directly sensing a fluid temperature using a conventional, non-acoustic, temperature sensor, a temperature sensing arrangement is provided which is insensitive to rapid temperature fluctuations and from which information related to the composition of the fluid can be derived and employed in the determination of the instantaneous fluid temperature based on the monitored acoustic velocity. Thus, the arrangement according to the present invention can be used in situations where the composition of the fluid is unknown or varies over time (i.e., over a time that is longer than the time constant of the conventional temperature sensor).
Preferably a value of the constant k is calculated as a time averaged value {overscore (k)} based on the equation (2) above from the signals received by the signal processor which are related to the acoustic velocity and the sensed temperature and updated periodically. This averaging mitigates any effects of rapid and transitory temperature changes in the fluid when deriving the information related to the fluid composition.
In the inventive method as well, by obtaining the value of the acoustic velocity in a fluid and obtaining the value of the sensed fluid temperature using a conventional temperature sensor, the instantaneous fluid temperature can be determined dependent on the obtained acoustic velocity value and the sensed fluid temperature even for a fluid of unknown composition.
The sensed fluid temperature is employed to derive information and preferably a time averaged value thereof obtained using the equation (2) above, related to the composition of the fluid which is then used to determine the instantaneous fluid temperature according to the obtained acoustic velocity.